a. Field of the Invention
This invention relates to a color cathode-ray tube (CRT) and, more particularly to a color CRT having a focus mask.
b. Description of the Background
A color cathode-ray tube (CRT) typically includes an electron gun, an aperture mask, and a screen. The aperture mask is interposed between the electron gun and the screen. The screen is located on an inner surface of a faceplate of the CRT tube. The screen has an array of three different color emitting phosphors (e.g., green, blue, red) formed thereon. The aperture mask functions to direct electron beams generated in the electron gun toward appropriate color emitting phosphors on the screen of the CRT tube.
The aperture mask may be a focus mask. Color CRT focus mask designs fundamentally incorporate at least two metallic electrodes separated by a suitable electrically insulating material and arranged in such a way as to create a periodic configuration of apertures through which electron beams pass on their way to the phosphor screen. When a suitable bias voltage is applied to the metallic electrodes, electric fields are generated at each of the mask apertures to form an electron optical lens, which provides the desired focussing of the electron beams upon the phosphor screen.
One type of focus mask is a tensioned focus mask, wherein at least one of the sets of metallic electrodes is under tension. Typically, for a tensioned focus mask, the vertical set of metallic electrodes is under tension, with the horizontal set of metallic electrodes overlying such vertically tensioned electrodes.
Where the two sets of metallic electrodes overlap, such electrodes are typically attached at their crossing points (junctions) by an insulating material. When a voltages is applied between the two sets of metallic electrodes of the mask, to create multipole focusing lenses in the openings thereof, high voltage (HV) flashover may occur. HV flashover is the dissipation of an electrical charge across the insulating material separating the two sets of conductive lines. HV flashover is undesirable because it may cause an electrical short circuit between the two sets of conductive electrodes leading to the subsequent failure of the focus mask.
Additionally, when the electron beams from the electron gun are directed toward the color emitting phosphors on the screen, redirected electrons (back-scattered electrons) from the phosphor screen may impinge upon the surface of the insulator material, causing it to become electrically charged. This surface charging modifies the desired potential field at the mask apertures and may impair the image quality displayed by the phosphor screen.
Thus, a need exists for an insulator material suitable for CRT focus masks that overcomes the above-mentioned drawbacks.
The present invention relates to a color cathode-ray tube having an evacuated envelope with an electron gun therein for generating an electron beam. The envelope further includes a faceplate panel having a luminescent screen with phosphor elements on an interior surface thereof. A focus mask, having a plurality of spaced-apart electrodes, is located adjacent to an effective picture area of the screen. The spacing between the first conductive metallic strands defines a plurality of apertures substantially parallel to the phosphor elements on the screen. Each of the first conductive strands has a substantially continuous insulating material layer formed on a screen-facing side thereof. A plurality of second conductive wires is oriented substantially perpendicular to the plurality of first conductive strands and are bonded thereto by the insulating material layer. The insulating material layer is partially or slightly conductive to an extent sufficient to prevent an accumulation of a significant electrical charge.